1. Field of Invention
The present invention relates to a dielectric lens, such as a Luneburg lens. More particularly, this invention relates to fabricating a dielectric lens having gaps in the dielectric material to provide an optimal permittivity distribution within the lens.
2. Discussion of the Prior Art
In the field of antenna engineering, lens antennas have many applications in the higher Radio Frequency (RF) bands, particularly in the microwave and higher portions of the electromagnetic spectrum Both the lens antenna and the reflector antenna are capable of producing a scanning beam without the motion of the lens or the reflector, or a motion of the entire antenna assembly. However, the lens antenna is more versatile than the reflector antenna in terms of producing wide-angle scanning beams.
A dielectric lens antenna, such as a Luneburg lens, is capable of producing a beam in any chosen direction by locating the feed at the focal point on the opposite side of the lens from the desired beam peak. In the case of the hemispherical Luneburg lens over a conductive plane, a beam peak may be produced within the hemisphere containing the lens. The hemispherical Luneburg lens is of interest for aeronautical applications due to its low profile and correspondingly low drag. The hemispherical Luneburg design may reduce the height of the antenna by as much as 50% for a given beamwidth.
For further background, the xe2x80x9cMathematical Theory of Opticsxe2x80x9d, written by R. K Luneburg, published by the University of California Press, Berkeley, 1964, discusses the theory of the Luneburg lenses applicable to this document.
Hemispherical lenses are generally discussed in xe2x80x9cFields and Waves in Communication Electronicsxe2x80x9d, written by Ramo, Whinnery, and Van Duzen, published by John Wiley and Sons, Section 12.19 Lenses for Direction of Radiation, pp.676-678.
The fabrication of Luneburg lenses is typically very costly. The use of conventional techniques to produce Luneburg lenses requires multiple shellsxe2x80x94each different from the others and manufactured to exacting tolerances. A technique has not yet been devised for the manufacture of such lenses that work acceptably well at frequencies at or above 44 GHz.
Most Luneburg lenses that exist today have been fabricated using the shell technique. Essentially, the Luneburg lens is fabricated from layers of concentric spherical surfaces. Each surface has a finite thickness and a slightly different index of refraction from the others such that the permittivity of the overall structure approximates the desired continuously varying index of the lens. This shell technique is commonly referred to as the xe2x80x9conionxe2x80x9d model method of fabrication. While the shell technique is effective in most low frequency terrestrial applications, it is unsuitable in high frequency aeronautical applications. In particular, the shells must be very thin and large in number to obtain good focusing at high frequencies. This makes the lenses complex and costly to produce. The large number of junctions between the surfaces results in discontinuities that reduce the gain of the antenna system formed from the lens. The materials used in these shell type lenses have problems with out-gassing at altitude and this can detrimentally alter the lens characteristics over time.
Fabrication of the lens from parallel slices has been proposed as a manner of solving the out-gassing problem and increasing the number of layers implemented. The principal problem with the slice technique is its cost. The slice technique requires that each slice be different from the other slices in the hemispherical lens if the slices are horizontal. As a result, a large number of different pieces are machined, assembled and laminated together at great cost.
The tapered hole approach has been proposed as a means of making low cost lenses. However, it has been found that the large hole diameter in the outer surface results in gaps that introduce excessive discontinuities particularly when the polarisation of the electric field is aligned with the length of the hole.
U.S. Pat. No. 5,677,796, issued to Zimmerman, discloses a method of constructing a spherical lens. Zimmerman teaches the use of a spheroid of uniform isotropic material that has a uniform dielectric constant throughout the fabrication process. Holes are drilled along a longitudinal axis extending radially from the centre of the lens in order to alter the lens dielectric constant. In a particular embodiment, Zimmerman adjusts the cross-sectional area of the holes to alter the dielectric constant of the lens. In contrast to the present invention, Zimmerman discloses the fabrication of the lens from a spheroid of uniform isotropic material, and not a sphere formed from identical wedges having identical permittivity distributions. Furthermore, the holes are drilled in order to alter the effective permittivity of the entire lens, as opposed to altering the permittivity of each individual wedge.
U.S. Pat. No. 3,470,561, issued to Horst, discloses a spherical dielectric lens constructed from a number of identical orange-slice shaped wedges. The wedges are fabricated from a dielectric material having a varying concentration of conductive slivers embedded inside each wedge. The concentration of the slivers in each wedge varies its dielectric constant in directions normal to the thickened edge of each wedge. Horst does not teach varying the dielectric constant of the individual wedges by drilling holes in a pattern into each orange-sliced wedge.
In view of the above shortcomings in the prior art, the present invention seeks to provide a dielectric lens that is fabricated from a number of identical wedges having specific patterns of gaps in the dielectric materialxe2x80x94i.e, patterns of holes. The number of wedges may be selected to achieve any desired approximation to the ideal Luneburg lens permittivity distribution. Finer discretization of the permittivity allows the lens to be used at higher operating frequencies.
The present invention provides a low cost method for fabricating a hemispherical or spherical Luneburg lens. The lens is manufactured from a number of identical elements, hereinafter termed wedges, where each wedge is defined by two planes having a common line which passes through the center of the lens. A plurality of holes, hereinafter termed gaps in the dielectric, are cut in each wedge at a position approximately normal to the radial direction of the lens. The position of the gaps in the dielectric alters the effective permittivity distribution within each individual wedge. Theses gaps are drilled, molded or produced by other means in a pattern on each wedge such that their permittivity varies radially so as to approximate the ideal permittivity distribution of a Luneburg lens. The gaps may have any shape, circular, square, or other. The permittivity can also be altered either by producing the gaps partially through the lens or all the way through the lens The gaps in the dielectric are essentially air voids, or voids filled with an alternative dielectric, that alters the effective permittivity of the lens. This has the particular advantage of ease of manufacture. In one embodiment, the gaps are produced offset to each other along different wedges in order to minimize discontinuities and resonances in the lens once the wedges are laminated together into a hemispherical or spherical structure.
In an alternate embodiment, the gaps in the dielectric may be constructed by cutting holes in the wedges and then filling these holes with material that has an alternative permittivity. The alternative permittivity may be lower or higher than that of the surrounding lens. The distribution of gaps is quite different in the two cases however. Filling of the gaps with material could minimize problems associated with ingress of moisture.
One advantage of the present invention is that the lens is fabricated from a number of identical pieces, thereby reducing the cost of manufacturing. Furthermore, the method of producing gaps in the dielectric into the individual wedges optimizes the permittivity of the entire lens. The end result is a hemispherical or spherical lens that operates at higher frequencies, and which has lower manufacturing costs than the methods of the prior art. Smaller hole sizes at closer spacing allow operation at higher frequencies. Furthermore, the present invention enables the lens to be fabricated with materials that do not have out-gassing problems.
In a first embodiment, the present invention provides a method of fabricating a dielectric lens including the steps of:
a) forming a plurality of wedges from a dielectric material, each of said plurality of wedges being substantially identical and orange-slice shaped, each of said plurality of wedges having two planar surfaces separated by an angular width, and each of said plurality of wedges having a plurality of gaps for altering an effective permittivity of said dielectric lens; and
b) assembling a hemispherical lens by connecting said plurality of wedges along said planar surfaces such that each of said planar surfaces intersect along a common line.
In a second embodiment, the present invention provides a dielectric lens including a plurality of wedges being formed from a dielectric material, each of said plurality of wedges being substantially identical and orange-slice shaped and including two planar surfaces separated by an angular width, and each of said plurality of wedges having a plurality of gaps for altering an effective permittivity of said dielectric lens, wherein said plurality of wedges form said dielectric lens by connecting said plurality of wedges along said planar surfaces such that each of said planar surfaces intersect along a common line.